Ask Ars: How much pee in a pool would kill you?

On Wednesday, we wrote about a scientific study showing that pee in a pool's chlorinated water can yield a toxic chemical called cyanogen chloride. That substance has recommended exposure limits from the World Health Organization and is also considered a chemical warfare agent.

The yields from the pool water in the study were not anywhere near deadly or even conclusively harmful. But the next question that bubbled up in Ars readers' minds was:

How much pee would it take to develop a deadly Olympic-sized swimming pool?

Further Reading

To figure that out, we extrapolated some figures cited in the paper (published in the American Chemical Society's Environmental Science and Technology) then cross-referenced those numbers with some real-world, commonly-accepted values. This yielded a ballpark answer… likely not quite the answer you'd expect. But, here we'll try to answer variations of the question in slightly more satisfying ways to provide a more comprehensive, academic understanding of urinating in pools.

First, a full rundown of how the experiment played out: a group of researchers added uric acid to chlorinated pool water. This combination resulted in the creation of cyanogen chloride, which can cause death in sufficiently high amounts. Again, the amount of cyanogen chloride generated in the experiment—roughly 20-30 micrograms per liter—is not considered immediately harmful and doesn't even meet the WHO's "danger zone" of 70 micrograms per liter or more. Nonetheless, it's there.

The concentration of uric acid used in the experiments was 5x10-5 moles per liter. This is actually a lower concentration than the standard excretions of a human bladder. Humans excrete between 1.2 and 4.5 millimoles of uric acid per day over an amount of urine that can range from 0.8 liters to two liters. To take the results to the limit, we'll take the highest concentrations of uric aid for the smallest amount of urine to get the highest reasonable concentration: 4.5 millimoles per 0.8 liters works out to be 5.625 millimoles of uric acid per liter of pee. Note that this is next-level pee concentration, as if for every glass of water you should be drinking, you ate a few strips of beef jerky instead.

Now, the 5x10-5 moles-per-liter concentration generated between 20 and 30 micrograms per liter of cyanogen chloride for the chlorine concentrations used in the experiment. The concentrations of chlorinated water vary throughout the experiment, but the concentrations that get that 20-30 micrograms per liter of cyanogen chloride are about 8-10 milligrams of chlorine per liter of water, roughly about two or three times the recommended chlorination of pool water.

According to the National Institute of Health, cyanogen chloride poisoning creates "mild effects" in humans at concentrations of 500-1,000 micrograms per liter. Beyond the concentration threshold of 2,500 micrograms per liter, it can cause "coma, convulsions, and death." (Delightful—these are just the figures we're looking for.)

As it turns out, the concentration of uric acid in pee is, to our calculation, about 112 times that of the uric acid concentration used in the experiment. If we could assume a proportional yield of cyanogen chloride just from using more uric acid, we could actually achieve toxic levels of cyanogen chloride for an Olympic pool of 10mg/L chlorinated water… for an equivalent quantity of urine. That means if each person is peeing 0.8L of the highly concentrated urine, their entire day's yield, into this pool, you'd need about three million people peeing in that pool. If you could get at that pool without dying of either suffocation or drowning in other people's urine, you could probably pull off death by cyanogen chloride poisoning or at least a pretty good coma.

However, there's a problem. The researchers in the paper showed that for a concentration of 0.33 millimoles of chlorine per liter (about 15 mg/L), the dilute concentration of uric acid (5x10-5 moles per liter) eliminated all of the free chlorine. Hence, if we want chlorinated water that can actually turn all of the uric acid we're peeing in it into cyanogen chloride, we need a more concentrated chlorinated solution.

If an approximately one-hundredth-strength-of-pee concentration of uric acid uses up 15mg/L chlorinated water, we need super chlorinated water-—on the order of 1500mg/L, or roughly half a liter of chlorine per liter of water.

In the end, we need a pool that is two parts water to one part chlorine and would probably burn the eyeballs out of your sockets and make your skin peel away from your bones (this calls for a pool boy who can only be criminally sadistic). If you and three million other people could get at this pool and unload your pee into it before your bodies melted, before the crowd crushed you to death, and before you drowned from the massive tidal wave of pee... yes, you could feasibly die of cyanogen chloride poisoning originating from chlorinated water and pee.